Field of the Invention
The invention relates to an absorption structure for absorbing neutrons, of the kind emitted by fuel assemblies of a nuclear reactor, for instance. The invention also relates to a method for producing such an absorption structure.
In order to achieve a desired neutron absorption, absorber parts in the form of various hollow shafts, canisters or similar configurations are typically produced, which surround a neutron-emitting object and thereby shield it. For instance, the use of such absorber parts permits the compact storage of neutron-emitting components, especially fuel assemblies, from nuclear reactors. In a fuel assembly storage frame which is known from Published European Patent Application 0 385 187 A1, absorber sheets, in the mounted state, form a number of hollow shafts that surround the fuel assemblies over their entire length. The hollow shafts formed of absorber sheets or absorber parts have a neutron-absorbing material, such as boron steel or boron-treated sheets.
The absorber parts are typically produced by introducing a certain proportion, for instance about 0.8 weight % of boron, into a molten parent metal. An absorber part made from that melt is then further processed into a hollow shaft, canister or other container. However, a disadvantage of that boron enrichment is that the parent material becomes brittle if boron above a certain concentration is smelted or incorporated into it, thus making further processing extremely difficult and even impossible in special applications.
Another possible way of producing absorber parts is to incorporate the neutron-absorbing material into a parent material, for instance of aluminum or plastic (plastic mat), and then to provide that parent material with austenitic sheets on both sides. However, as a result it cannot be assured in every case that there is a homogeneous distribution of boron within the parent material. The demand for a homogeneous distribution of the neutron-absorbing material within the absorber part must be met due to the risk of a criticality accident in nuclear reactors. Moreover, it must also be assured that dehomogenization within the absorber part will not occur, even later.
U.S. Pat. No. 4,218,622 describes a composite neutron absorber part, which has a thin carrier film or sheet, onto which a polymer matrix into which boron carbide particles have been incorporated, is applied. Fiberglass is preferably used as the material of the carrier film or sheet. The boron carbide particles are distributed uniformly on the surface of the polymer matrix, and a boron concentration of up to 0.1 g/cm.sup.2 is achieved. When the composite absorber part is used in a fuel assembly storage basin, that absorber part has a thickness of up to 7 mm, is constructed in the form of a film or a sheet, and is suspended between an inner wall and an outer wall. The extent to which a homogeneous distribution of the boron carbide particles disposed on the surface of the polymer matrix is assured over a relatively long period, especially with a view to possible abrasion at the surface, cannot be learned from U.S. Pat. No. 4,218,622. The problems of a homogeneous distribution of neutron-absorbing material, both in terms of surface area and volume, are still largely present in the absorber part mentioned in U.S. Pat. No. 4,218,622.